1. Field of the Invention
The present invention relates to a process for the preparation of a dibromopropyl ether compound having a high melting point.
2. Description of the Related Art
It is known that a dibromopropyl ether compound represented by the following general formula: ##STR2## is a flame retardant. For example, Japanese Examined Patent Publication No. 50-35103 and Japanese Examined Patent Publication No. 62-49900 teach that a bis-dibromopropyl ether of tetrabromobisphenol-sulfone (TBS) and a bis-dibromopropyl ether of tetrabromobisphenol-propane (TBA) are valuable as a flame retardant for plastics. Furthermore, Japanese Examined Patent Publication No. 63-39585 and Japanese Examined Patent Publication No. 57-289 disclose processes for the preparation of these compounds.
More specifically, Example 5 of Japanese Examined Patent Publication No. 63-39585 discloses a process in which bis-(4-allyloxy-3,5-dibromophenyl)sulfone is dissolved in methylene chloride and reacted with bromine, methylene chloride is evaporated and a treatment is carried out at 105.degree. C. under a reduced pressure (3 mmHg) to obtain a vitreous bis-dibromopropyl ether of TBS having a softening point of 50.degree. to 53.degree. C. Furthermore, in Japanese Examined Patent Publication No. 57-289, it is taught that a bis-dibromopropyl ether of TBA. is obtained by brominating 2,2-bis-(4-allyloxy-3,5-dibromophenyl)propane and this compound is a resinous product melting at 40.degree. to 50.degree. C. The reason why the melting point or softening points of the bis-dibromopropyl ether compounds of TBS and TBA are low and are about 40.degree. to about 50.degree. C. is that these compounds are not crystallized. It is taught in the foregoing patent publications that, if these compounds are recrystallized from solvents or are crystallized by adding poor solvents to solutions of these compounds in good solvents, products having a high melting point can be obtained.
When these compounds are used as flame retardants for plastics, in general, these compounds are powdered and mixed into pellets and the compositions are processed, but since the softening points are low, the following disadvantages arise:
(1) Where the compound is pulverized, the pulverization must be carried out at a low temperature not exceeding 15.degree. C.
(2) Since the powder is readily blocked during transportation or storage, refrigerated transportation or storage in a refrigerated warehouse is necessary.
(3) When the compound is mixed with pellets of plastics, since the compound is fused in a Henschel mixer, an unsatisfactory mix is often obtained.
(4) When flame-retardant pellets are formed by using an extruding molding machine, bridging occurs in a hopper and a plastic material is not smoothly supplied.
(5) Since the softening points of plastics are greatly different from those of the compounds, the compounds are melted prior to the plastics in an extrusion molding machine, and therefore, the compounds slip and the biting becomes poor, with the result that a satisfactory mix is not obtained.
(6) When flame-retardant pellets are prepared by mixing the compounds into plastics, it is necessary to use plastics in the powder form and to use a twin-screw extruder.
Because organic solvents are used therein, the processes disclosed in Japanese Examined Patent Publication No. 63-39585 and Japanese Examined Patent Publication No. 57-289, that is, the process in which recrystallization from ethylene glycol monomethyl ether is carried out for obtaining a product having a high melting point, and the process in which methanol is added in a methylene chloride solution of the dibromopropyl ether compound to effect crystallization, are defective in the following points:
(1) Filtration and drying steps are necessary, and there is a risk of fire and problems of safety and hygiene. Moreover, it is necessary to use an apparatus for recovering the solvents.
(2) In the case of the bis-dibromopropyl ether of TBS, methylcellosolve and ethylcellosolve are excellent as the recrystallization solvent, but the recrystallized product must be recovered by filtration and dried. Since the boiling points of these solvents are high, at 124.degree. C. and 134.degree. C., respectively, heating is necessary for the drying, and as the solvent is contained in the crystallized product, a remelting of the crystallized product is caused by this heating. Therefore, after the filtration, it is necessary to replace the recrystallization solvent by an inert solvent such as methanol, and to dry the recrystallized product.